1. Technical Field
The present invention relates in general to an improved data communications networking system, and in particular to an improved local area network (LAN) switch. Still more particularly, the present invention relates to a multi-port LAN switch for providing a communications link between a network adapter and a network concentrator, a network adapter and another network adapter or a network concentrator and another network concentrator.
2. Description of the Related Art
In digital data transmission systems, composite clock and data signals in binary form are transmitted over media such as wires or fiber optic cables from a transmission line transmitter to a transmission line receiver. The transmitter and the receiver in a data communications system may each be a single computer or may each comprise a local area network (LAN) of computers. An individual computer or station in a LAN may both send information to other stations in the LAN and receive information from other stations. The station inserts into the LAN when it desires to communicate with another station in the LAN, and detaches from the LAN when the communications are complete.
A common LAN topology is the "token-ring" network. The token-ring is used to interconnect the devices attached to the network. The token-ring network allows unidirectional data transmission between stations in a ring-like circuit by a token passing procedure. The ring topology permits tokens to be passed from a node associated with a particular attached device, such as a personal computer, to another node in the ring. A node that is ready to send data can capture the token and thereafter insert data for transmission. If information received by a node or station is destined for a station further along the network, the receiving station must pass the information along the LAN to the next adjacent station, and so forth, until the information reaches its final destination. A device or computer station attempting to gain access to a node of the token-ring will have an adapter, which is physically connected to the token-ring. This accessing device must carry out a procedure following a standard protocol in order to access the token-ring.
One type of token-ring product has two data transmission speeds, 4 Mbps and 16 Mbps. Both of the transfer speeds are frequently used, and often, the data transmission speed of 4 Mbps may be used in one network, while the data transmission speed of 16 Mbps may be used in another network, both of which a user may wish to access.
Many LANs employ concentrators, or hubs, also known as multi-station access units, to connect many stations at a single network node. These multistation access units connect individually with each station along a 4-wire cable called a lobe. Multiple lobes extend out from a concentrator to individual stations to form a star-like structure. Physically, each station is individually attached to the concentrator through its lobe where it may access the network node. All stations attached to a particular concentrator operate at the same network speed (e.g., 4-Mbps). When the concentrator is connected to a token-ring network, the logical configuration of the network places each station connected to the concentrator at a separate node within the ring. A concentrator can individually connect the attached devices in a token-ring, or it may be connected with other concentrators to form a larger token-ring comprised of all the devices attached to all concentrators. An intelligent concentrator is one that includes processor controlled switching electronics for controlling access to the network.
A concentrator is usually referred to as a "Multi-Station Access Unit" or MAU. Such system is provided for in the IEEE 802.5 specification, which refers to such system as a "Trunk Coupling Unit." Single lobes comprised of two twisted-pair wires connect a network adapter or other communication device to a port of the concentrator. Single lobes are combined with other identical lobes to form a complete concentrator. While the number of lobes in a concentrator can vary, the most popular configuration utilizes eight lobes and such is due primarily to the physical size of the token-ring connector as it fits in a standard equipment rack.
The function of a MAU is to electrically insert and remove a workstation or personal computer from a communication networking system, or more specifically, to connect or remove a workstation or personal computer from a token-ring network. Control of the insertion or removal of a workstation from a token-ring network is accomplished, as specified in the IEEE standards, by means of a DC voltage that is sometimes referred to as a "phantom drive current." This phantom drive is applied between the two pairs of conductors in the data cable or lobe that connects the workstation to the Multi-Station Access Unit. When the phantom drive current (or voltage) is present at a preselected level or potential, the MAU functions to insert the workstation into the network. When the phantom drive is absent or falls below a preselected level, the workstation is removed from the network. For a more detailed reference to information relating to the operation of a MAU, reference may be made to IEEE 802.5. In addition, a full-duplex (FDX) adapter can gain access to the network by sending an FDX registration frame to a port on a LAN switch. The FDX frame is a special frame identifying an adapter or switch port as having FDX capability. If an adapter or switch port receives this frame, it responds by transmitting its own FDX registration frame. After this FDX frame handshaking occurs, the phantom drive is asserted. This method of insertion is described in U.S. Pat. No. 5,561,666, Ser. No. 339,267, filed Mar. 6, 1995, entitled "Apparatus and Method for Determining Operational Mode for a Station Entering a Network.", Chorpenning, J., et. al., incorporated herein by reference.
Each computer attached to the network is connected to a respective lobe port of the concentrator via a cable, and the computer exercises control of the insertion/bypass mechanism via the cable using the phantom drive. This DC voltage is transparent to the passage of computer-transmitted data, hence the name "phantom". The impressed voltage is used within the lobe port of the concentrator to affect the serial insertion of the computer in the ring. Cessation of the phantom drive causes a de-insertion action that will bypass the computer and cause the computer to be put in a looped (wrapped) state.
A computer attached to the network contains a network adapter card having the electronics and hardware necessary to both connect with a MAU via a lobe and to insert and communicate in a token-ring network. In existing token-ring adapters, convention requires that data be transmitted on the orange/black pair of wires and received on the red/green pair of wires in the medium interface cable. Token-ring adapters connect directly to a MAU, such as the IBM.RTM. 8228, so that the MAU receives data on the orange/black pair of wires and transmits on the red/green pair of wires. The phantom drive current is asserted by the token-ring adapter on the orange/black pair of wires to allow insertion into the token-ring. The phantom drive current provides a dual function of both detecting faulty wiring and engaging the relay in the MAU to serially connect the computer into the token-ring. Thus, by convention, adapters source phantom drive and a MAU sinks phantom drive.
While the above described token-ring network allows every network adapter to communicate with every other network adapter, this communication must be performed over the token-ring network through a concentrator. Consequently, two computers situated adjacent to each other within the LAN must communicate using the network's limited bandwidth, which is shared with every adapter attached to the network There are two problems that prevent direct connection of two network adapters. First, the direct connection of two network adapters meeting the standard set forth in IEEE 802.5 would have their transmit twisted-pair (orange/black pair) directly connected, and their receive wires (red/green twisted-pair) directly connected, preventing any communication between the two network adapters. Second, currently available network adapters cannot sink phantom drive; they only source phantom drive. Without the capability to sink phantom drive, an adapter attempting to communicate would source phantom drive to the receiving adapter, which could not sink the phantom drive. Thus, the sourcing adapter would detect a "wire fault" condition. Upon detecting a wire fault condition, the adapter would automatically stop transmitting data.